1. Field of the Invention
The present invention relates to a ventilator/respirator for regulating the flow of air to, and the pressure of air in, the lungs, having a pressure gauge for determining the pressure of air in the lung and a volume-determining device for determining the volume of air supplied to the lung.
2. Description of the Prior Art
In a healthy lung, air passes during inspiration (inhalation) through airways and bronchi to the alveoli in the lung. An exchange of gas occurs in the alveoli, whereby blood is oxygenated while carbon dioxide is simultaneously transferred to air in the alveoli. During expiration (exhalation), air enriched with carbon dioxide is extracted from the lung, permitting the entry of fresh air. Since a healthy lung has large compliance, i.e., flexibility, a relatively large volume of air can be inhaled every time a person takes a breath without excessive increases in the pressure of air in the lung. Efficient exchange of oxygen and carbon dioxide is essential to the load on the heart and to the supply of vital oxygen to the body. When gas exchange becomes impaired, the load on the heart increases because the heart has to pump more blood through the lungs.
As a result of injuries or disease, the function of the lung can be affected to such a degree that a life-threatening condition could develop. For example, the alveoli might collapse, impairing or, at worst, preventing the essential exchange of gas between air in the alveoli and blood in the lungs capillaries. Another possibility is that the compliance of the lung could decline so much that an insufficient volume of air is inhaled in each breath. Connecting the damaged lung to a ventilator/respirator may then be necessary to keep the patient alive. A ventilator/respirator can subject the lung to an air pressure large enough to open the alveoli so sufficient gas exchange can take place. The operation of such a device for opening the lung and maintaining it open is described in an article entitled "Open up the lung and keep the lung open" by B. Lachmann, Intensive Care Medicine (1992) 18:319-321. Air at a relatively high pressure must be supplied to the lung in order to open the alveoli, whereas a much lower pressure is needed during ventilator/respirator breathing to keep the alveoli open once they have been opened. At the same time, the risk of lung trauma in forced respiration increases at higher pressures (barotrauma) and/or large volumes of breathing gas (volutrauma), especially if lung compliance is simultaneously poor.
Another consideration in this context is that a patient on a ventilator/respirator must be weaned off the unit for a certain period of time before normal, spontaneous breathing resumes. The requisite weaning time can be considerably shortened if the forced respiration is performed in such a way that it does not impose needlessly heavy loads on the lung with high absolute pressures, large changes in pressure and/or large volumes of air/breath. Non-traumatic treatment of a lung also promotes healing, which is obviously advantageous to the patient. Faster healing also conveys major economic benefits, especially in intensive care etc. where beds are very expensive.
Modern ventilators/respirators operate according to various ventilation principles which strive to keep a lung completely open by creating some form of positive pressure in the lung. This can be accomplished, e.g., by having the ventilator generate a positive end expiratory pressure (PEEP) or by reducing the relative duration of expiration in order to create a positive pressure in the lung at the end of expiration (intrinsic PEEP), both methods keeping the alveoli open. A third option is to increase the breathing rate, thereby retaining a surplus of air in the lung and creating a positive pressure which keeps the alveoli open. The latter method greatly depends on the ratio between the duration of inspiration and expiration.
Another factor to be taken into account, making it harder for the physician to select a treatment method for a patient, is the circumstance that all lungs are individual and respond to treatment in different ways, thereby affecting the pressure at which the lungs are opened and the pressure required to keep the lungs open. In addition, every individual lung's responses to its prevailing condition varies. Adapting treatment accordingly is especially important when a sick or damaged lung is healing.